How can i keep my teeth straight after orthodontist without a retainer?

No retainer: If you don't wear a retainer after ortho treatment, your teeth may not remain 'perfect'. Ask your orthodontist to place a 'permanent retainer that is bonded to the lingual surfaces of your anterior teeth. Why would you or your parents pay all that money if you don't wear the retainer? If you are doing invisalign, final positioner is often your retainer. Please discuss this with your orthodontist.

You can't: A retainer by definition, retains the position of your teeth. Absent it's use, teeth will tend to revert to the position they had before braces.

You can't: To invest all that time, effort, and money only to throw it away is very sad. The human body changes constantly. As an adult you have a whole new skeleton every 7-8 years. If you don't retain the orthodontic result, it will destabilize, perhaps in a few years requiring re-treatment (and retention).

Be very lucky: Historical evidence suggests that retention is a required stage of orthodontic treatment for most if not all patients. In our world, perfection does not maintain itself, it requires effort, especially with something you use every day. Consider your retainers as maintenance devices for your smile and bite.